The following description is used to describe the background and products, materials and production methods that may comprise specific parts of preferred embodiments in the disclosure. The floor types described below typically have two major common features.
They are intended to be installed in a floating manner with a mechanical locking system and the decor is, in most cases, obtained by colour pigments.
The different flooring types described below use two major classes of polymers, thermosetting and thermoplastic polymers, to provide a bonding between particles, mainly wood and mineral fillers. Such fillers are used to improve mechanical properties or just as extenders to decrease the amount of polymers used.
Thermosetting polymers are characterized in that they melt, float and polymerize with increased stiffness when heat and pressure is applied. They are completely cured and cannot be melted again. Thermoplastic particles on the other hand soften and melt when temperature is increased and the stiffness increases when they are cooled down.
Thermoplastic polymers are very moisture stable and may be formed as a product comprising only a specific polymer material. Thermosetting polymers are moisture sensitive and very brittle. They must be combined and reinforced with fibres, generally with wood fibres.
1. Thermosetting Flooring
This flooring category is characterized in that thermosetting resins such as, for example, Melamine Formaldehyde (MF), Urea Formaldehyde (UF) or Phenol Formaldehyde (PF) resins are combined with mainly wood fibres and cured under heat and pressure such that separate core or surface layers may be formed.
a) Laminate Floors
The majority of all laminate floors have a wood design. Such laminated floors have a core of 6-12 mm fibreboard, a 0.2 mm thick upper decorative surface layer of laminate and a 0.1-0.2 mm thick lower balancing layer. The decorative and wear properties are generally obtained with two separate layers of melamine formaldehyde impregnated paper (a decorative layer and a wear layer), one above the other. The decorative layer is a printed paper and the wear layer is a transparent overlay paper, which comprises small aluminium oxide particles. The core is generally a high-density fibreboard (HDF) comprising wood fibres and a mix of MF and UF resins. The decorative paper and the overlay paper are directly laminated by a hot-hot production method to the HDF core under high heat and pressure (170° C., 40 bar). Hot-hot pressing generally refers to a pressing method where a press table has essentially the same closing temperature as the opening temperature (e.g., within about 10° C.). Hot-hot pressing is very cost efficient since no cooling of the press table is needed but can only be used in direct lamination of thermosetting based surfaces that cure at high heat and that are laminated to rather thick core materials comprising thermosetting resins that are able to absorb the steam from the moisture of the core and the surface layer that is created at high pressing temperatures above 100° C.
A separate laminate sheet may also be produced in a continuous or discontinuous pressing operation and the laminate sheet is thereafter glued to a core material. This gluing, that may be made in cold condition, allows that separate laminate sheets may be bonded to different core materials, provided that they have similar moisture movements as the surface layer when relative humidity (RH) varies between dry (RH 20%) and wet conditions (RH 90%).
b) Powder-Based Floors
Recently new “paper free” floor types have been developed with solid surfaces comprising a substantially homogenous powder mix of fibres, binders and wear resistant particles hereafter referred to as WFF (Wood Fibre Floor).
The powder mix may comprise aluminium oxide particles, thermosetting resin such as melamine formaldehyde resins and wood fibres. In most applications decorative particles such as, for example, colour pigments are included in the mix. In general, all these materials are applied in dry form as a mixed powder on a wood-based core such as a HDF core and cured in a hot-hot direct lamination step under similar heat and pressure as used in laminate floors.
Digital powder printing has been developed and it is possible to create very advanced wood, stone and tile designs by injecting water based ink drops comprising pigment dispersions into the powder prior to pressing. A recently developed Binder and Powder (BAP) printing method may also be used to provide a digital print on a powder-based surface. Conventional digital printing methods are divided in two separate steps comprising a separate application of a liquid binder, generally referred to as “blank ink” and of dry colorants, generally referred to as “dry ink”. Coloured particles of dry ink are applied on a panel surface. Some particles are bonded by a digitally formed pattern of blank ink applied by a print head. Other non-bonded dry ink particles are removed and the remaining bonded particles form a digital decor. This two-step process may be repeated and several colours of dry ink may be applied such that an advanced multi colour high definition digital print may be formed in a cost efficient way. The major advantage is that rather simple water based liquid substances may be used and the pigment based dry ink colorants are very cost efficient since no pigment dispersions are needed.
2. Thermoplastic Flooring
This flooring category is characterized in that thermoplastic materials such as, for example, polyvinyl chloride (PVC), polypropylene (PP), or polyethylene (PE) are mixed with mineral or wood fibres. The final pressing is made under lower pressure than generally used in, for example, laminate production and with a hot-cold pressing method. The temperature is generally similar, about 140-160° C. Thermoplastic material melts at high temperatures and no curing takes place. The press table or belt must therefore be cooled down below 100° C. such that the thermoplastic layer is converted from a floating paste to a solid layer. Separate thermoplastic layers may be fused or laminated together by heating and cooling under pressure.
c) LVT Floors
Luxury Vinyl Tiles, generally referred to as LVT floorings, are generally constructed as a layered product made of thermoplastic material such as PVC, generally referred to as vinyl, mixed with mineral fillers, colour pigments and plasticizers. The name LVT is somewhat misleading since a major part of LVT floors have a plank size with a wood design. They may also comprise other plastic materials that are not vinyl. LVT has become a generic name for high quality resilient floor that may be installed floating with a locking system.
Thermal moulding is used to form the PVC layers. Co-rotating twin-screw extruders are used for the production of high-quality thermoforming PVC layers with fillers. The screws press and mix the material under heat.
The extruder may be filled with pre-formed small plastic pellets comprising a compound of PVC, pigments, fillers etc. Compounding is a process where mixing of polymers and additives in a molten state under heat and generally also under pressure is used in order to form plastic pellets comprising a homogenous blend of the different raw materials. Co-kneaders, twin-screw extruders and internal mixers are the most common compounders used in the plastic industry.
The compounding process may be avoided and an extruder may also be directly filled with basic plastic particles and separate fillers and other additives that are mixed in a hopper attached to the extruder. This method may be used to save material cost. Advanced twin-screw extruders with optimized screw geometry combined with accurate temperature control are used to obtain high melt homogeneity.
The extruder comprises a flat sheet die that provides a paste with essentially the same width as the finished layer. The die has generally oil temperature-controlled adjustable die lips that provide a wide thickness range.
The melted paste is further processed with 3-roll polishing calenders comprising a fixed central hot roll and two adjustable rolls on each side for calibration. The first roll is hot and the third roll is cold. The melt feed angle, which is determined by the weight and elongation of the melt, can be changed by adjusting the position of the first roll.
Horizontal, inclined or vertical roll arrangements may be used.
A wide range of downstream equipment and methods are used to laminate the layers under heat and pressure and to cool the plastic layers to a multi-layer sheet. One simple method is to cut the layers to sheets and to use a multi opening hot-cold press. In continuous production several extruders may be used and the layers are hot when they are laminated together, in most cases with an intermediate layer of glass fibre. Cooling and stabilization of the panel in order to avoid warping takes a lot of space and a continuous production line may be several hundred meters long.
Extruders and calenders may be replaced with a continuous double belt presses comprising heating and cooling under pressure combined with several nip rollers. Plastic compounds in pellet form are scattered on the lower belt and pressed together under heat and pressure. A similar continuous press may also be used to laminate several sheets and foils to a LVT floor panel.
The core is made primarily of several layers that comprise PVC mixed with chalk and/or limestone fillers. The most common core materials comprise a middle part with about 50% limestone and a balancing layer with about 40% limestone. The lower limestone content in the backing is used to balance the high plastic content in the top layer. Some LVT core material may comprise up to 80% limestone fillers. Such core materials with high filler content are very brittle and it is difficult to form a strong locking system.
A 0.1 mm high quality printed decorative PVC foil is applied on the upper side of the core. A transparent wear layer of vinyl with a thickness of 0.1-0.6 mm is generally applied on the decorative foil. The base layer, the decorative foil and the transparent layer are fused or laminated together with heat and pressure in continuous or discontinuous press operations to a 3-6 mm floor panel. A transparent UV curing polyurethane (PU) lacquer is generally applied as a protective layer on the transparent PVC foil. LVT floor are installed floating with a locking system similar to laminate and powder based floors.
The decorative effects are obtained with a decorative foil that is printed separately and fused on the core layers. The foil comprises a single colour that generally is white and covers the dark colour of the core. The foil provides a base colour for the rotogravure printing process where special solvent-based inks comprising pigments are used to create wood, tile and stone designs.
d) WPC Floors
Wood Plastic Composite (WPC) panels comprising thermoplastic material mixed with wood particles have been used in extruded sections and injection mouldings in several industries during many years. Recently WPC flooring panels in plank sizes have been developed for outdoor use mainly as decking. Thermoplastic material such as PVC or PP is mixed with wood in the form of sawdust or wood chips by wood particle weight (wt) of about 50% to 80%. Products intended for exterior use may comprise 50 wt % or less of wood particles. The wood content may exceed 70 wt % in products intended for interior use. The processing is similar to the methods used to produce a LVT floor. A two-step process may be used where wood fibre particles and plastic particles in a first step are compounded together under heat and pressure to plastic pellets and fed into an extruder. Fibres and plastic granules may also be mixed directly during extrusion. Alternatively compounded plastic pellets may be scattered on a belt and pressed to WPC sheets in a continuous double belt press comprises a heating and a cooling section as described above. The decorative properties are mainly obtained by pigments that are included in the wood/plastic mix. WPC panels may also be printed, painted or foiled.
3. Combi-Floor
Thermoplastic and thermosetting materials may be combined. Thermoplastic LVT layers may, for example, be glued to a HDF core or a WPC core.